How important was the concept of the frontier to America's diplomatic development ?

If author William Earl Weeks is to be believed, the frontier's effect on America's diplomatic activities was far from positive. In a review of Weeks's book Building the Continental Empire: American Expansion from the Revolution to the Civil War, Michael A. Morrison of the Purdue University Department of History commented on what he saw as the brutal violence and lies of "virtuous farmers" against Native Americans, Europeans and Mexico:

Weeks clearly demonstrates that empire-building came at a cost. . . .A peaceful nation of independent, virtuous farmers warred on Seminoles, banished Native Americans from their lands, bullied and lied to European nations (and in the process compromised its own integrity), beat up Mexico, and conspired with filibusters of all stripes in order to realize its destiny. In a dark irony, these peaceful yeomen, unable to agree on the ends of expansion in the late 1850s, settled the issue by slaughtering each other by the tens of thousands.

This, of course, is in stark contrast to the lofty and romantic ideals that developed around the phrase "manifest destiny", coined by American columnist John Sullivan in 1845 when he wrote that America had a "divine destiny" to expand wherever possible and create a nation embracing "moral dignity and the salvation of man". As noted in Wikipedia/eNotes, Weeks also argued that three themes emerged again and again among proponents of westward expansion in general, and the concept of "manifest destiny" in particular:

the virtue of the American people and their institutions;

the mission to spread these institutions, thereby redeeming and remaking the world in the image of the U.S.; and

the destiny under God to do this work.

These things, then, were a convenient way to justify the need for war with Mexico to acquire Texas as well to create tension and conflict anytime a coveted territory was at stake (California, New Mexico, Oregon Territory, where ever Native Americans were). Whether or not Weeks is correct in his abrasive assessment of American expansionism may be a matter of opinion, but most people would probably not disagree with the idea that expansionism during the 1800's (and even into the 1900's) didn't exactly get the nation off to a promising start in terms of future international diplomatic relations.